The present invention relates to the use of additives to improve the reversibility, in the presence of propylene carbonate, of a carbon electrode of a lithium ion secondary electrochemical generator.
It particularly applies to the production of specific high-power secondary generators, which are of great interest for the development of portable devices and, on a more long-term level, for the production of electric vehicles.
At the present time, for these applications, the choice seems to focus on lithium ion secondary electrochemical generators using a carbon compound, particularly graphite, as a negative electrode.
Indeed, graphite is currently the most promising carbon since, in its lamellar structure, it can accept up to one lithium atom for six carbon atoms, at potentials close to that of metallic lithium. However, lithium can only be inserted into graphite in a reversible manner in a few appropriate electrolytes. In this way, it is difficult to use propylene, in spite of its good properties, in such generators, since it has the disadvantage of decomposing in the graphite structure and thus preventing the insertion of lithium in the graphite electrode.
A. N. Dey and B. P. Sullivan reported this problem of the decomposition of propylene carbonate in the graphite electrode, in J. Electrochem. Soc., 117, 1970, pages 222-224, [1]. Similarly, M. Arakawa and J. Ichi Yamaki reported this problem and proposed a propylene carbonate decomposition schedule in J. Electroanal. Chem., 219, 1987, p. 273-280, [2].
To, overcome these difficulties, D. Billaud, A. Naji and P. Willmann proposed in J. Chem. Soc., Chem. Commun., 1995, pages 1867 and 1868, [63], to subject the graphite electrode to a pre-treatment to form on said electrode a protective coat impervious to propylene carbonate molecules but enabling the diffusion of lithium ions. This protective coat is formed by electrochemical reduction, in an electrolyte comprising ethylene carbonate and lithium perchlorate.
Another solution to avoid this disadvantage was described by Z. X. Shu, R. S. McMillan, J. J. Murray and I. J. Davidson in J. Electrochem. Soc., 143, No. 7, July 1996, p. 2230-2235, [4]. In this case, an electrolyte mixture of chloroethylene carbonate and propylene carbonate is used.
The first solution described in reference [3] has the disadvantage of requiring an electrode pre-treatment step, before the electrode is used in the secondary electrochemical generator with a propylene carbonate-based electrolyte.
The second solution described in reference [4] has the disadvantage of requiring a significant proportion of additive since chloroethylene carbonate, chloro-EC, with the formula: 
generally represents 50% by volume of the electrolyte, and this proportion of chloro-EC must not be less than 30%.
The precise purpose of the present invention is to improve the reversibility of the insertion of lithium in a carbon electrode of a lithium ion secondary electrochemical generator using an electrolyte comprising propylene carbonate, which makes up for these disadvantages.
According to the invention, the process to improve the reversibility of the insertion of lithium in a carbon electrode of a lithium ion secondary electrochemical generator, using an electrolyte comprising propylene carbonate, consists of forming electrochemically, during the first use of the generator, a passivation film on the surface of the carbon electrode essentially using an organic compound selected from the cyclic and non-cyclic xcex1-halogenated esters.
These organic compounds are of particular interest since they make it possible to form a passivating coat on the carbon electrode, during the first use of the electrochemical generator, while being used in a much lower quantity than that required in the case of chloroethylene carbonate used in reference [4].
According to the invention, a xcex1-halogenated ester that is liquid at ambient temperature or soluble in propylene carbonate PC or in a mixture of PC and other solvents, and is also stable under the operating conditions of the electrochemical generator, is selected.
The halogen used in the ester may particularly be chlorine, bromine or iodine.
Examples of such non-cyclic xcex1-halogenated esters include alkyl halogen formiates and halogen acetates, particularly alkyl chloroformiates and chloroacetates.
Chloroformiates comply with the formula ClCOOR where R represents a linear or ramified alkyl group, preferably a lower alkyl group, with one to four carbon atoms such as the methyl or ethyl groups. In particular, it is possible to use methyl chloroformiate.
Alkyl chloroacetates comply with the formula ClCH2-COOR where R has the same significance as above.
Other non-cyclic esters liable to be used may comply with the formula R-CHCl-COOR where the Rs may be the same or different, with the same significance as above.
According to the invention, it is also possible to use cyclic xcex1-halogenated esters complying with the following formula, for example: 
wherein X represents a halogen such as Cl, Br or I, and n is a whole number ranging from 1 to 3, it being possible to replace one or more of the CH2 groups by methyl, ethyl, propyl or butyl groups.
An example of a cyclic ester of this type is xcex1-bromo-xcex3-butyrolactone BrBL which complies with formula (II) given above, where n=2 and X represents Br.
According to a preferred embodiment of the invention, the organic compound selected from the cyclic and non-cyclic xcex1-halogenated esters is added to the electrolyte of the electrochemical generator, so as to enable the formation of the passivation film on the surface of the electrode during the first use of the generator.
According to a second embodiment of the invention, the organic compound selected from the cyclic and non-cyclic xcex1-halogenated esters, is adsorbed on the carbon electrode. In this case, the electrode is first of all subjected to an organic compound adsorption step by immersing it in said compound, before using it in the electrochemical generator. As described above for the first use, a passivation film on the carbon electrode is formed from the adsorbed compound by electrochemical reduction.
In the first embodiment of the invention, the electrolyte advantageously comprises a solution of at least one lithium salt in a solvent composed of a mixture comprising propylene carbonate and the organic compound selected from the cyclic and non-cyclic xcex1-halogenated esters.
The lithium salt(s) used may be selected from the salt generally used in lithium ion electrochemical generators. Lithium perchlorate LiClO4, lithium hexaf luoroarseniate LiAsF6, lithium hexaf luorophosphate LiPF6, lithium tetrafluoroborate LiBF4, lithium trifluoromethanesulfonate (lithium triflate) CF3SO3Li and lithium trifluorosulfonylimide (LiTFSI) LiN(CF3SO2)2 are preferably used.
In an electrolyte of this type, the added organic compound content may be low. It is selected so as to form a passivation film with a sufficient thickness over the entire surface of the electrode to prevent contact between the propylene carbonate molecules and the carbon electrode, but this proportion must not be excessively high since, if the film is too thick, it will prevent the diffusion of the lithium ions in the electrode.
When xcex1-bromo-xcex3-butyrolactone is used as the added organic compound, it may represent 0.5 to 3% by volume of the solvent composed of the mixture of propylene carbonate and xcex1-bromo-xcex3-butyrolactone, and preferably 1 to 2% by volume of the solvent.
The lithium salt concentrations of the electrolyte according to the invention are generally such that the electrolyte contains at least 0.1 mol/l of lithium ions. For example, the lithium salt concentration may be within the range from 0.1 mol/l to saturation.
In the lithium ion electrochemical generator using such an electrolyte and a carbon electrode, the other electrode, or positive electrode, may be made of various materials such as oxides, sulphides or oxysulphides.
Examples of oxides that may be used include vanadium oxide V2O5, nickel oxide NiO2, cobalt oxide CoO2, mixed. cobalt and nickel oxides, manganese oxides, molybdenum oxide MoO3, chromium oxides and vanadium bronzes MxV2O5, where M represents iron, sodium, potassium, lithium, silver, aluminium, chromium, barium, nickel or cobalt.
Examples of sulphides that can be used include titanium sulphide TiS2, molybdenum sulphide MoS2 and mixed nickel and molybdenum sulphides.
Examples of oxysulphides that can be used include molybdenum and titanium oxysulphides.
In an electrochemical generator of this type, using an electrolyte according to the invention, a separator is generally placed between the electrodes and said separator may be composed of a microporous film made of polypropylene or polyethylene, for example.
The generator. may be produced in, the form of a cylindrical generator comprising a spiral winding of the two electrodes separated by the separator if required. It may also be produced in the form of a prismatic type generator with plane electrodes facing each other and possibly a separator placed between these electrodes.
Other characteristics and advantages of the invention will be seen more clearly upon reading the following description, which is naturally given as a non-restrictive illustration with reference to the appended drawings.